This invention relates generally to a method for applying a conductive material to stator bars, and more specifically, to methods for applying a conductive tape to liquid-cooled stator bars for high-voltage generators.
In at least some known high-voltage generators, a stator yoke surrounds an armature core and partially encloses a plurality of stator bars, which are sometimes referred to as “stator windings” or “armature windings”. At least some known stator bars include a plurality of strands of copper conductors that are wound in the armature to form loops. The stator bars are generally positioned in slots of the power generator so that desired voltage and current characteristics can be generated during operation. High-voltage insulation is wrapped around the stator bars to maintain ground insulation between the conductors and the stator core and other grounded objects.
While in the slot of the power generator, stator bars are subject to a cross-slot flux produced by the normal load current. If the stator bars are loose within the slots, vibration or “bar bouncing” caused by the magnetic forces can damage the stator bars. Thus, in order to reduce this motion, the final size of the stator bars must be carefully configured so that the bar is tightly wedged into the slot.
At least some known methods for fabricating stator bars include adding a conductive material or coating in order to prevent corona discharges between the stator bar and stator core laminations. Corona discharges deteriorate the high-voltage insulation which leads to premature failure of the stator bar. Thus, material having a lower resistance than the insulation is applied to the outer surface of the insulation in order to prevent or limit the corona activity.
In one known method, particularly for after-market liquid-cooled stator bars, electrical insulation is first applied around the stator bar. The insulation is then cured in a vacuum-pressure autoclave. After curing in the autoclave, the bar is inspected to confirm that it satisfies dimensional requirements. A conducting adhesive is then applied to the cured ground insulation, followed by wrapping the coated insulation with a glass tape or other material (e.g., fabric, felt, or mat). Typically, the width of the tape is approximately one and a half inches wide, making the process of wrapping the bar labor intensive. After the adhesive and the tape dries, which takes approximately eight hours, a conductive paint is applied to the glass tape. The conductive paint helps obtain the desired surface resistivity for the final stator bar. The paint takes approximately eight hours to dry. In some cases, the bar is held (e.g, on sawhorses) which requires the bar to be moved so that those areas covered by sawhorses can be painted. Another eight hours of curing time is then needed.
After the paint cures, the stator bar is then inspected again to verify the bar is appropriately sized. Furthermore, the stator bar can be surface resistance tested, in order to ensure the surface resistivity is within an appropriate range. Bars are then “high potential” proof tested at elevated voltages relative to operation in order to identify any flawed ground wall insulation.
However, the armoring process described above can be labor intensive. Also, the process requires substantial time in curing the paints and adhesives applied to the stator bars (approximately 16 hours) and substantial time in manually sizing the bar and applying the armor. Lastly, a significant amount of bar handling is required due to the number of process steps for armoring and this presents the potential for handling damage to occur.
Overall, the above process can be inefficient and costly. Furthermore, generators may include a variety of stator bar configurations and/or require a variety of stator bar sizes. As mentioned above, it is important to control the finished size of the bar to prevent them from being loose in the slot. Thus, alternative methods and more cost-efficient methods for manufacturing stator bars and for applying a conductive material are desired.